Stents are widely used for supporting a lumen structure in a patient's body. For example, a stent may be used to maintain patency of a carotid artery, coronary artery, other blood vessel or other body lumen such as the ureter, urethra, bronchus, esophagus, or other passage. A stent is typically a metal, tubular structure, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, metallic, or other materials and stents may elute over time substances such as drugs.
In certain stent designs, the stent is an open-celled tube that is expanded by an inflatable balloon at the deployment site. Another type of stent is of a “self-expanding” type. A self-expanding stent does not use a balloon or other source of force to move from a collapsed state to an expanded state. A self-expanding stent is passed through the body lumen in a collapsed state. At the point of an obstruction, or other deployment site in the body lumen, the stent is expanded to its expanded diameter for its intended purpose. An example of a self-expanding stent is a coil structure that is secured to a stent delivery device under tension in a collapsed state. At the deployment site, the coil is released so that the coil can expand to its enlarged diameter. Coil stents can be manufactured using a variety of methods, such as winding of wire, ribbon, or sheet on a mandrel or by laser cutting from a tube, followed by the appropriate heat treatments. Other types of self expanding stents are closed-cell or open-celled tubes made from a self-expanding material, for example, the Protëgé GPS stent from ev3, Inc. of Plymouth, Minn. Cellular tube stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods. The shape, length and other characteristics of a stent are typically chosen based on the location in which the stent will be deployed.
Conventional stents generally are comprised of struts or wires having openings therebetween. Some workers in the field have added coverings to stents and thereby substantially occluded the stent openings. Examples of such devices include covered stents, stent-grafts, and mesh covered stents. Generally speaking, covered stents and stent-grafts can be used as conduits for fluids in situations where little or no fluid loss through the wall of the conduit is desirable. Generally speaking, mesh covered stents can be used to maintain a luminal diameter large enough to permit flow in the conduit yet prevent liberation of emboli from the wall of the lumen into the lumen.
One delivery technique for a self expanding device such as a stent, covered stent, stent graft, or mesh covered stent is to mount the collapsed device on a distal end of a device delivery system. Such a system can be comprised of an outer tubular member and an inner tubular member. The inner and outer tubular members are axially slideable relative to one another. The device (in the collapsed state) is mounted surrounding the inner tubular member at its distal end. The outer tubular member (also called the outer sheath) surrounds the device at the distal end. One or more portions of the device are releasably attached to the inner tubular member by means of interlock assemblies and the interlock assemblies facilitate proper positioning and control of the device during device deployment.
Prior to advancing the device delivery system through the body lumen, a guide wire is first passed through the body lumen to the deployment site. The inner tube of the delivery system is hollow throughout at least a portion of its length such that it can be advanced over the guide wire to the deployment site. The combined structure (i.e., device mounted on device delivery system) is passed through the patient's lumen until the distal end of the delivery system arrives at the deployment site within the body lumen. The deployment system should have good bending flexibility in order to traverse tortuous vessels encountered during system advancement to a treatment site, and the device may include radiopaque markers to permit a physician to visualize positioning of the device under fluoroscopy prior to deployment. At the deployment site, the outer sheath is retracted to expose the device. The exposed device is free to self-expand within the body lumen. Following expansion of the device, the inner tube is free to pass through the device such that the delivery system can be removed through the body lumen leaving the device in place at the deployment site.
In prior art devices very high forces can be required to slide the inner and outer tubular members relative to one another and thereby deploy the device. This is especially true when attempting to deploy, for example, long devices, devices with a large amount of expansile force when compressed in a sheath, or bulky devices compressed into small sheaths in tortuous vessels.
What is needed is a system that permits low deployment force of long devices, devices with a large amount of expansile force when compressed in a sheath, or devices compressed into small sheaths in tortuous vessels.
In addition, in prior art devices, the stent may prematurely deploy as the outer tube is retracted. Namely, with the outer tube partially retracted, the exposed portion of the stent may expand resulting in the remainder of the stent being squeezed out of the outer tube. This can result in the stent being propelled distally beyond a desired deployment site. Also, once the stent is partially unsheathed, it is sometimes determined that the stent placement needs to be adjusted. With some existing systems, this is difficult since the stent has a tendency to force itself out of the sheath thereby making adjustments difficult. Further, once the stent has been deployed, subsequent adjustment of the stent deployment location can be difficult because re-sheathing typically cannot be readily accomplished. To overcome some of these problems some stent delivery systems are comprised of interlocks on the stent and on the inner member. See for example U.S. Pat. No. 6,814,746 to Thompson et. al., entitled “Implant Delivery System With Marker Interlock”, and U.S. Pat. No. 6,623,518 to Thompson et. al., entitled “Implant Delivery System With Interlock”, the contents of both included herein in their entirety by reference.
While interlocks on stents and stent delivery systems have improved the precision with which some stents can be delivered to treatment sites, it has been found that certain types of stents are not effectively delivered even by the existing interlock systems. For example, long stents, thin stents, or stents with a large amount of expansile force, when compressed in a sheath, tend to buckle along their length as the outer sheath is withdrawn from the inner tubular member. Stents with high axial flexibility parallel to the central axis of the stent can also be very difficult to deploy, and to deploy precisely, with the existing interlock systems. Visualization of these stents during deployment, particularly of the long stents, is often inadequate. These types of stents can also be difficult to load into the stent delivery system as well.
What is needed is a system that permits easy loading, precise delivery, and good visualization during deployment of long stents, thin stents, stents with a large amount of expansile force, and stents with high axial flexibility.